The systems and methods described herein pertain to the production of petroleum products and natural gas, and particularly to compositions effective as anti-agglomerate low dosage gas hydrate inhibitors (“LDHI's”) for the prevention of gas hydrate plugs.
Gas hydrates are solids that may form during hydrocarbon production, in particular in pipelines and other equipment, that may impede or completely block flow of hydrocarbons. These blockages not only decrease or stop production, potentially costing millions of dollars in lost production, but are also very difficult and dangerous to mediate. Unless properly handled, gas hydrates may explode, rupturing pipelines, damaging equipment, endangering workers and putting at risk the ocean environment.
Gas hydrates may form when water molecules become bonded together after coming into contact with certain “guest” gas molecules. Hydrogen bonding causes the water molecules to form a regular lattice structure that is stabilized by the guest gas molecules. The resulting crystalline structure precipitates as a solid gas hydrate. Guest molecules can include any number of molecules, including carbon dioxide, methane, butane, propane, hydrogen, helium, freons, halogens, and noble gases.
Thermodynamic, anti-agglomerate, and kinetic inhibitors are three general classes of hydrate inhibitors. Thermodynamic inhibitors are most commonly used. Thermodynamic inhibitors, such as methanol and ethylene glycol must typically be used at high concentrations to be effective, concentrations that may present environmental concerns. For instance, methanol is used in concentrations of up to 50% methanol to water ratio, with glycol as much as 30% glycol to water. Methanol presents other challenges, as it is flammable and can be corrosive. Thus, thermodynamic inhibitors are often not appropriate for many drilling operations, particularly environmentally-sensitive drilling operations.
Kinetic inhibitors and anti-agglomerate inhibitors typically function at lower concentrations than thermodynamic inhibitors and are therefore termed LDHI's. Kinetic hydrate inhibitors are polymers that may prevent or delay the nucleation of hydrates. Thus the kinetic hydrate inhibitors limit hydrate crystal size and growth such that hydrate plugs are not allowed to form in tubular goods. However, kinetic hydrate inhibitors are capable of handling only low-to moderate subcooling—typically subcooling of about 10-25° F. (subcooling is the difference between the operating temperature of the hydrocarbon system and the temperature at which hydrates would form at the same operating pressure). Thus, kinetic hydrate inhibitors may not be suitable in deep and ultra-deep wells, where subcooling may be greater than 30° F.
Anti-agglomerate gas inhibitors are typically more cost effective than thermodynamic inhibitors, as they may be used in much lower concentrations and are typically useful in environments with greater subcooling than would be appropriate for kinetic inhibitors. However, many of the traditional anti-agglomerate LDHI's contain residual halides, such as HCl, HBr, and the like, and residual organic halides. Residual halides have been know to cause corrosion and stress corrosion cracking (“SCC”) in metal piping and production equipment. One example of a commonly used anti-agglomerate LDHI is quaternary anti-agglomerates containing residual organic halides, such as Kelland, 2006. As an example, Milburn et al. U.S. Pat. No. 6,444,852 entitled “Amines Useful in Inhibiting Gas Hydrate Formation,” which is hereby incorporated by reference in its entirety, describes anti-agglomerate ether-containing amine compounds that are quaternized with a halide. Especially in the case of organic halides, they can be very toxic and environmentally unfriendly. This is particularly true when the inhibitors are applied continuously. In addition, traditional anti-agglomerate LDHI's may break down and become less effective when exposed to high temperatures above 250° F.
What is needed is an anti-agglomerate LDHI that does not contain residual halides in sufficient quantities to present an inappropriate risk of corrosion or stress cracking and that is less toxic and more environmentally friendly than the traditional LDHI's.